The present invention relates to retaining rings and specifically to retaining rings formed of a composite, e.g., epoxy/graphite, material for supporting the field end windings of the rotors of rotating electrical devices such as generators, motors and condensers.
The field end windings of a generator, motor, condenser, or other rotating electrical devices are normally supported by a steel retaining ring fitted over the rotor end. The inner diameter of the retaining ring is less than the outer diameter of the rotor, typically in the range of 40 to 50 mils. To secure the retaining ring to the rotor end, the retaining ring is heated for a shrink fit onto the rotor end. Heat shrinking the retaining ring, however, can damage the insulation of the field end windings or the retaining ring itself or both. A centering ring is then fitted into a central bore of the retaining ring at the end away from the shrink fit. The retaining ring and the rotor cannot be keyed to one another because of the retaining ring's inability to withstand the stresses encountered during operation.
When a rotating electrical device is disassembled for maintenance or repair, the retaining ring must be heated in order to remove it from the rotor. Because of the required heavy shrink fit of the retaining ring to the rotor end, it is difficult and costly to supply the heat necessary for disassembly. The insulation on the windings can also be damaged during this process. With excessive heat, the retaining ring itself may be damaged. In the prior art, when a retaining ring is partially damaged because of stress corrosion, a cylindrical intermediate spacer ring has been inserted between the salvaged retaining ring and the rotor. But due to fit-up problems associated with the retaining ring, the cylindrical intermediate spacer ring, and the rotor, the retaining ring is typically completely replaced with a new retaining ring instead of being salvaged. Also, due to tolerance variations of the rotor end, retaining rings are usually customized. Further, because of the necessity of applying heat to retaining rings for both assembly and disassembly, repairs to rotating electrical devices at remote locations, such as ships at sea, for example, are especially difficult to perform.
In my above-mentioned prior patent, there is provided a system for assembling an end retaining ring on and disassembling it from the end of the rotor body without use of heat or heat shrinking techniques. In that system, there is provided an auxiliary ring disposed about the rotor body end and having an outer tapered surface. The retaining ring has at one end a complementary tapered surface such that the retaining ring end may be disposed about the tapered surface of the auxiliary ring. The auxiliary ring includes a fluid line having a fluid inlet port and a fluid outlet port opening into an annular groove about the outer tapered surface of the auxiliary ring whereby fluid may be disposed between the tapered surfaces. By supplying fluid under high pressure between the tapered surfaces through the fluid line, assembly and disassembly of the retaining ring onto the auxiliary ring is facilitated and accomplished without use of heat or heat shrinking techniques.
Both the present invention and my above-mentioned patent solve the problems associated with using heat shrinking techniques of the prior art by eliminating the need for such techniques. Moreover, both permit removal of the end retaining ring of the electrical device to more easily enable repairs, for example, remote pairs in naval generators, provide assembly and disassembly at substantially reduced costs and time and avoid damage to the surrounding insulation on the end windings of the rotor. While the retaining ring system of my prior patent is eminently useful and practical, there remains, however, the ever-present danger of stress corrosion cracking, the relatively high costs of manufacture and replacement of the retaining ring and other practical considerations of its use with very large diameter rotating electrical devices, such as generators. The present invention particularly enables the employment of even larger diameter generators because of the materials of the retaining ring hereof, eliminates stress corrosion problems, and affords other advantages as will become apparent.
According to the present invention, it has been found desirable and advantageous to replace the costly, non-magnetic alloy steel retaining ring with one formed of high strength, high stiffness, less expensive, less dense, and lighter weight composite material. Particularly, the field end windings support system hereof utilizes a primary retaining ring, an auxiliary ring, and an inner retaining ring. More specifically, the primary retaining ring of the present invention is formed of a composite, e.g., epoxy and graphite, preferably graphite fibers wound on a bias about a mandrel or form to form a generally cylindrical housing open at one end and closed at its opposite end. When forming the primary retaining ring hereof, the auxiliary ring is integrally formed with it and about its open end. Thus, the graphite fibers with impregnated epoxy are wound about the form or mandrel, preferably on a bias, to integrally incorporate therein the auxiliary ring adjacent the open end of the primary retaining ring. The auxiliary ring has an annular interior tapered surface which mates with a complementary tapered surface formed along the outer surface of the inner retaining ring. The inner retaining ring has an inner diameter providing for a line-line or a light press-fit on the rotor body end and overlies the radially outwardly opening grooves or slots at the end of the rotor body and through which ends the field end windings project. The inner retaining ring thus supports the windings in the slots, while the end windings projecting therefrom are supported by the primary composite retaining ring.
Composite retaining rings formed of epoxy and graphite are not per se new. For example, retaining rings formed of this composite material are discussed in a paper titled "Fabrication and Evaluation of Scale Model Graphite/Epoxy Retaining Rings for Generator Applications," by Dr. J. F. Meier. However, previous attempts at using composite materials for generator retaining rings were thwarted and complicated by the requirement to lock the ring to the generator rotor in order to prevent axial movement.
To assemble or disassemble the primary composite retaining ring on the rotor end in accordance with the present invention, a fluid line is provided in the auxiliary ring, the fluid line having a fluid inlet port for receiving high pressure fluid. The inlet port may be disposed to receive fluid externally of the rotor or from within the body of the end windings, i.e., from adjacent the axis of the rotor. The complementary tapered surfaces of the auxiliary ring and the inner retaining ring have axially spaced seals defining therebetween an annular groove in communication with a fluid outlet port of the fluid line in the inner retaining ring. Consequently, by applying high pressure fluid to the complementary tapered surfaces between the retaining rings, the integral composite assembly of the primary retaining ring and auxiliary ring may be readily and easily assembled and disassembled relative to the inner retaining ring.
With the windings in the slots being supported by the inner retaining ring, the end windings extending from the rotor are supported by the primary composite retainer ring. Axial support is provided by the radially inwardly directed flange at the closed end of the primary retaining ring opposite the auxiliary ring. Interposed between that flange and the end windings stack is a spring. This spring accommodates thermal expansion between the primary composite retaining ring and the end windings. The spring thus facilitates the axial differential expansion of the copper end windings and the composite retaining ring. More particularly, as the copper end windings become hot, and because the composite is a poor insulating material and remains cold, the copper end windings are displaced axially outwardly toward the end retaining ring. The spring disposed therebetween accommodates this movement without unbalancing the rotor. At rotor speed, however, the composite has a higher coefficient of expansion, thus the spring must accommodate the difference in axial expansion of the copper end windings and the composite flange of the primary retaining ring. Preferably, the spring is formed of a fiberglass material in the form of corrugations emanating from a central axis. That is, the ridges and grooves of the corrugations extend radially outwardly from the axis of the spring and lie circumferentially spaced one from the other about the spring. Thus, the spacing between the grooves and ridges becomes larger the further the grooves and ridges are located away from the axis of the spring. The spring is essential for, among other reasons, to prevent axial skewing of the axial end windings. This enables the end windings to remain balanced, thus inhibiting mechanical vibrations at rotor speed.
In accordance with the present invention, there is also provided a short-circuiting ring carried along the interior wall of the primary retaining ring. Particularly, the short-circuiting ring radially overlies the amortisseur windings. In this manner, there is provided a substantially continuous circumferential electrical path for the amortisseur windings to accommodate a fault. The composite ring is protected from differential radial expansion of the short-circuiting ring by a circumferentially extending corrugated metal spring ring overlying the outside diameter of the short-circuiting ring. The corrugations preferably extend axially. A thin metal shield overlies the corrugated metal spring between the composite ring and the corrugated metal spring ring.
In a preferred embodiment according to the present invention, there is provided a system for supporting the end windings of a rotatable electrical device having a rotor body rotatable about an axis and a rotor body end, comprising a primary retaining ring having an open end mountable to the rotor body end and extendable beyond the rotor body end to support the end windings, the primary retaining ring being formed of a composite of epoxy and graphite, an auxiliary ring carried by the primary retaining ring adjacent its open end and being integrally secured thereto, the auxiliary ring having a radially inwardly directed surface tapering radially inwardly from the open end of the primary retaining ring toward the opposite end thereof, and an inner retaining ring for overlying the rotor body end and having a radially outwardly directed surface tapering radially inwardly toward the axis in a direction toward the opposite end of the auxiliary ring. The tapered surface on the inner retaining ring is generally complementary to and for mating engagement with the tapered surface of the auxiliary ring.
In a further preferred embodiment according to the present invention, there is provided a system for supporting the end windings of a rotatable electrical device having a rotor body rotatable about an axis and a rotor body end, comprising a retaining ring assembly having an open end mountable to the rotor body and extendable beyond the rotor body end to support the end windings, the retaining ring assembly being substantially closed at its opposite end, means for securing the retaining ring assembly on the rotor body end and a spring carried by the retaining ring assembly adjacent an end thereof opposite the open end for accommodating axial expansion and contraction of the end windings relative to the retaining ring assembly.
In a still further preferred embodiment according to the present invention, there is provided a system for supporting the end windings of a rotatable electrical device having a rotor body rotatable about an axis and a rotor body end, comprising a primary retaining ring having an open end mountable to the rotor body end and extendable beyond the rotor body end to support the end windings, an auxiliary ring carried by the primary retaining ring adjacent its open end and being integrally secured thereto, the auxiliary ring having a radially inwardly directed surface tapering radially inwardly from the open end of the primary retaining ring toward the opposite end thereof, an inner retaining ring for overlying the rotor body end and having a radially outwardly directed surface tapering radially inwardly toward the axis in a direction toward the opposite end of the auxiliary ring, the tapered surface of the inner retaining ring being generally complementary to and for mating engagement with the tapered surface of the auxiliary ring and a spring carried by the primary retaining ring adjacent an end thereof opposite the open end for accommodating axial expansion and contraction of the end windings relative to the primary retaining ring.
Accordingly, it is a primary object of the present invention to provide a novel and improved end retainer ring assembly for a rotary electrical device such as a generator, wherein the retaining ring is formed of a high strength, high stiffness, inexpensive, less dense composite material which precludes sensitivity to stress corrosion cracking and eliminates completely any need for using heat shrinking techniques to assemble it on and disassemble it from rotary electrical devices.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.